[sdiy] Precision power regulators for low noise circuits?

[rob]

Hi,
If you want to extract the best of CEM3340 VCO then use reference
voltage chip to generate the power for the chip itself or at least the
frequency sensitive areas such as the frequency pots. 

A precision VCO cannot be run from standard 3-pin LM7XXX voltage
regulators, these devices are okay for producing the basic voltage rails
in an analog synthesizer but they are too unstable to directly run an
accurate VCO from. In the 1970's the LM723 was an excellent performer
giving 1mV drift on hand selected chips. It was therefore used in a
variety of analog synthesizers including the E-mu Systems Audity (along
with a LM399 to give some added temperature stability) and it pops up in
older DAC designs too. However IMHO this cheap chip is not sufficient
for a modern VCO. 

The next step up is a dedicated voltage reference chip such as the
Analog Devices or Maxim REF01 with a typical price in the UK of £2.50.
They are often used as a reference voltage for DAC chips. However these
don't out perform the humble LM723 by very much and they should be
ignored for modern precision VCO designs. What we need is a precision
voltage reference chip such as the; 

Maxim MAX6175A (SMT only) 
Linear Technology LT1019A 
Linear Technology LT1027 

The LT1027 has better temperature stability and is the one I use. You'll
usually need to buffer the output with a low drift Op Amp. A dual Op Amp
will also create the negative voltage reference. These chips are not
cheap - typically £7.50 each, but the results are audible. The E-mu
Systems VCO that I have recreated uses these precision voltage
references and it performs as well as many modern VCO designs, except of
course for the legendary MOTM VCO...

VCO frequency and temperature accuracy needs to be approached
holistically, so its important to use precision components in other
areas of the VCO circuit such as: 0.1% 15ppm resistors in critical areas
such as the control voltage inputs up to the expo generator. And the 1%
polystyrene caps mentioned on the list, make sure they are close to the
chip.

Similarly better Op Amps can be used than the TL. I use LT1012/1013 for
CV summing and OPA2134/134 for Audio paths. However there are quite a
few different high grade Op Amps to choose from, but beware HF
oscillation.

If you want some proven modular style PCB's for your CEM (or SSM's)'s
then checkout my web site. There is a range of VCO, ADSR, Filters, etc.

Regards
Rob
www.emulatorarchive.com



Hello SDIY,

I have a few CEM chips to work with and wanted to try and pull the most 
accurate performance I can from them.
I wanted to ask of any recommendations on a precision voltage regulator 
IC, if theres something better than the standard 7815s out there.

Also any recommendations on precision opamps I should check out, fast 
slew rate? Is there anything better than TL072 ect.

Thanks,

chris k